One form of magnetic separation device which functions by magnetizable particle entrapment is generally referred to as a High Gradient Magnetic Separator or HGMS. An HGMS comprises a liquid-permeable, cylindrical canister containing liquid-impermeable packing elements of magnetizable material between the canister inlet and outlet. The packing material may be paramagnetic or ferromagnetic and may be in particulate or filamentary form, for example, it may comprise wire wool, wire mesh, knitted mesh, or steel balls. The packing may be in the form of a single block which essentially fills the canister or it may be in other forms, for example, concentric cylinders or rectangular plates. The term "matrix" is generally employed to refer to the packing. The term is used by some in the industry to refer to the totality of the packing; however it is commonly used, as herein, to refer to individual elements of the packing (i.e. concentric cylinders, rectangular plates, etc.).
The canister is surrounded by a magnet which serves to magnetize the matrix contained therein, the magnet generally being arranged to provide a magnetic field in the direction of the cylindrical canister axis. With the matrix magnetized, a slurry of fine mineral ore, for example clay dispersed in water, is fed into the inlet of the canister. As the slurry passes through the canister, the magnetizable particles in the slurry are magnetized and captured on the matrix. Eventually, the matrix becomes substantially filled with magnetizable particles and the rate of capture decreases so that the quantity of magnetizable particles in the treated slurry leaving the outlet of the canister reaches an unacceptably high level. The slurry feed is then stopped and the canister matrix rinsed with water to remove all non-magnetic material from the matrix. During the wash step, the magnetic field acting on the matrix is reduced to a sufficiently low value to enable the magnetizable material to be washed off the matrix elements with a high speed stream of water. The magnetic field may be reduced by de-energizing the magnet. HGMS systems operated in this way are referred to as switched HGMS systems.
It is generally recognized that de-energizing, washing, and subsequent re-energizing is, however, inefficient as regards cycle time and power consumption. Accordingly, an arrangement has been developed in which the magnet does not have to be de-energized to permit matrix regeneration. Instead, two matrix canisters are provided and moved alternately into the magnetic field of the separation zone. Thus, as one matrix canister is engaged in separation, the second can be flushed and the matrix regenerated. HGMS systems operated in this way are referred to as reciprocating canister HGMS systems or RCHGMS systems.
The magnetic field required for a switched HGMS or an RCHGMS can be provided by an electromagnet operating at ambient temperatures, a permanent magnet, or a super-conducting magnet operating at cryogenic temperatures (cryogenic magnets). Cryogenic magnets for use with switched HGMS or a RCHGMS in industrial applications include a close coupled helium liquefaction system which has sufficient cooling power to maintain the magnet coil below the critical super-conducting temperature. The coil is held in a reservoir of liquid helium which may be surrounded by one or more radiation shields, the whole being contained in a cryostat vessel. The shields are maintained at low temperatures by refrigeration means which may include cooling pipes for circulating liquid nitrogen and/or cryocoolers. See, e.g., U.S. Pat. Nos. 5,743,410 and 5,759,391 to Stadtmuller.
A radial flow canister design is described in U.S. Pat. No. 4,079,002 to Iannicelli, the present applicant. Matrix elements, either rectangular elements spaced parallel to one other or annular elements spaced concentrically, are arranged within a highly intense magnetic field, and means are provided for establishing flow of the mixture to be separated in parallel flow paths through the matrices. The device may be used in both wet and dry separation processes. For example, the device is useful for separating particles of impurities from an aqueous slurry (for example, iron-mineral contaminates from an aqueous slurry of crude kaolin clay), for the purification of industrial minerals such as calcium and magnesium carbonates, asbestos, zircon, bentonite and talc, for the beneficiating of metal oxides, and for the treatment of coal (such as the removal of pyrites during desulfurizing). A major drawback of this configuration, however, is that cumbersome and difficult steps of placing and compressing the annular or rectangular mats are required to prepare the radial flow matrix.
The initial development and commercialization of stainless steel wool as a material for magnetic separator matrix elements involved manual packing of bundles of stainless steel tow into a canister to achieve a desired volume packing (usually 2 to 6%). This method suffered from the disadvantage that the matrix had a non-uniform density, which often created non-uniform flow through the matrix as the slurry passed preferentially through less-dense packing and voids. Furthermore, the matrix was susceptible to deformation and could be altered during high velocity flushing, making the method unreliable and unworkable for production-scale use.
Later developments involved the use of steel mats or felt made by laying down layers of steel wool tow, which could be obtained by scraping wire during its manufacture. The layers of tow wire were laid on a table and then interlocked by punching with needles with barbs resembling a straightened fish hook. Initially, the square mat was laid by shifting the orientation of alternate layers 90.degree. before punching. This produced non-uniformities which were clearly apparent by holding up the layers to light. Subsequently, uniformity of the mat was significantly improved by laying rows of tow on a circular table matching the diameter of the finished pad. The table was rotated a given amount (30.degree., for example) after each row was laid. Building up the layers to about 0.5 inch produced a visually uniform mat after punching. Besides giving a more uniform and stronger mat, the table-made mat avoided wasted scrap because each layer of tow was matched to the diameter of the rotated table. In the orthogonal method, a square mat was laid down and cut into a circle when completed. The four comers were usually scrapped.
The mats so-produced, however, only contained about 2% steel wool by volume. In order to increase the density of the mats to the required 6 or 8% metal by volume, a stack of pads was compressed while in the canister by either placing weights (several tons) on the canister top or by using a hydraulic press capable of exerting 50 or more tons pressure. Unfortunately, such high compression of the matrix placed a high stress on the canister and often cracked the perforated plates of the design above and below the matrix. Indeed, the canister itself would frequently crack and leak. To achieve a matrix density of 6% following compression, a stack of about 100 pads had to be installed into the canister one or two at a time, which presented a risk of stretching and distorting the pads. The process also required hours of down time and at least three workers. Yet another disadvantage was the risk of injury to the workers from the compression process and from handling sharp steel wool.
Compression of the matrix against perforated plates additionally created corrosion problems at the interface of the steel wool pad and the perforated plate. Perforated plates at the entrance and exit of the canister perform two diverse functions in prior art HGMS units. First, perforated plates sandwich, compress, and restrain the steel wool matrix which has a spring back force of up to 50 tons for a large magnetic separator. Second, perforations in the plate serve to distribute flow of clay slurry or water across the cross-sectional area of a canister. A typical perforated plate consists of 1/4 inch thick 430 stainless steel having a regular pattern of 1/4 inch holes, giving about 50% open area. Slurry or water pumped from a plenum through the perforated plate enters the matrix as a series of small jets or streams. After flowing through several inches of steel wool matrix, these discrete streams coalesce into a uniform plug flow across the cross-sectional area of a canister. The non-plug flow space adjoining the matrix, both at the entrance and the exit of a canister, has dead spots and is difficult to clean of magnetizable products, non-magnetizable products, and minute debris present in the slurry.
As a result of this gradual accumulation of deposited particles in the first few inches of canister (equivalent to 6 or more layers of matrix), corrosion followed by partial plugging of the matrix occurs. This process is intensified by electrolytic action existing between the perforated plate and steel wool (even though they are of similar alloy composition) and the galvanic action between debris coatings and the matrix.